1. Field of the Invention
The present invention relates to a variable magnification optical system for projection and a projection-type display apparatus. For example, the present invention relates to a small-sized variable magnification optical system for projection appropriate to magnify and project rays carrying video image information from a light valve onto a screen, and to a projection-type display apparatus on which the variable magnification optical system for projection is mounted.
2. Description of the Related Art
In recent years, projection apparatuses (projection-type display apparatuses) using DMD (digital micromirror device: registered trademark) display devices instead of liquid crystal display devices, as light valves, became utilized. In the projection-type display apparatus, the DMD controls the reflection direction of light output from a light source by using rectangular micromirrors. The micromirror has high reflectance, and the inclination of the micromirror is changeable in a range of about 10 degrees or more based on a received video signal. The DMD makes projection of the video image possible by collecting only desirable reflection light on a screen. This is made possible, for example, by vertically and horizontally arranging several million or more mirrors on a substrate, and by performing digital control on all of them independently from each other. Each minor corresponds to a pixel in each video image.
The DMD display device differs from the liquid crystal display device in that illumination light does not need to be polarized. Therefore, a loss in light amount is small, and the DMD display device is excellent also in accurately expressing gradation. Hence, in the projection-type display apparatus using the DMD display device having such advantages, a lens for projection with excellent optical characteristics is requested so that sharp and highly precise images corresponding to the DMD display device are obtainable.
Further, as a demand in mobile use has increased, and the price of the projection-type display apparatus has become lower, reduction in size, weight and cost tends to be requested also in the lens for projection. Further, a wider angle of view, which makes projection from a shorter projection distance onto a large display screen in a small room possible, is requested. Further, it is desirable that the magnification of the lens for projection is changeable so that the size of an image projected on the screen is changeable.
It is necessary to simplify the lens structure of a lens for projection to reduce the size and weight of the lens for projection. As variable magnification optical systems for projection with simple group structure consisting of a small number of lenses, optical systems disclosed in Japanese Unexamined Patent Publication No. 2008-107798 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2010-113150 (Patent Document 2) are known. Patent Documents 1 and 2. disclose zoom lenses, in which an entire system consists of five lenses, and a first lens group having negative refractive power and a second lens group having positive refractive power are arranged in this order from a magnification side. These two lens groups move in the direction of an optical axis during magnification change.